1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a computer readable storage medium having stored therein an image processing program.
2. Description of the Related Art
Digital watermarking, for example, has been known as a technology for ensuring the security of a document printed on a paper. Digital watermarking is a technology for embedding confidential information and the like into an image There are various types of digital watermarking, such as, a technology for superimposing a dot pattern converted from confidential information onto a background of an image (document image) representing a document so that the confidential information can be embedded into the document image, and a technology for forming a void (cut) and a spot (dot) on an edge portion (e.g., a character portion) in the document image or finely shifting a space between characters in the document image so that the confidential information can be embedded into the document image (contents processing technology). However, the conventional digital watermarking has a problem in that the confidential information may not appropriately be embedded depending on a document. For example, in the method of embedding the dot pattern into the background of a document image, when the document image contains a number of character regions, diagram regions, and photograph regions or when the background of the document image is colored, it may be difficult to embed the dot pattern into the document image or to read the confidential information from the dot pattern. Furthermore, in the method using the contents processing technology, when the document image contains only a small number of character regions and diagram regions or when the document image contains a number of photograph regions (gradation images), because the numbers of character portions and edge portions are small, it may be difficult to embed the confidential information into the document image. On the other hand, Japanese Patent Application Laid-open No. 2008-131420 discloses a technology in which, when the confidential information cannot be embedded into the document image by the digital watermarking because the document image contains a number of character regions, a barcode encoded from the confidential information is superimposed onto a specific region, such as a region containing a small number of characters, in the document image so that the confidential information can be embedded into the document image.
However, in the technology disclosed in Japanese Patent Application Laid-open No. 2008-131420, when the document image contains a large number of character regions such that it contains virtually no region having a small number of characters, a user may not be able to read the contents of the document image because of the superimposition of the barcode. Therefore, the original function of the document image may be lost. Thus, it is difficult to solve the problem that the confidential information cannot appropriately be embedded depending on a document. Furthermore, in the technology disclosed in Japanese Patent Application Laid-open No. 2008-131420, the barcode is embedded into a region containing a small number of characters, i.e., a region considered to be less important in the document image. Therefore, when the barcode is blacked out or cut out, the confidential information can easily be invalidated. Thus, it is difficult to ensure the security.